Method for enhanced inductive coupling to plasmas with reduced sputter contamination

ABSTRACT

A method of gas plasma treating a workpiece in a process chamber having RF coil outside the chamber, a flat dielectric window, and a electrically conducting shield, adapted to be located between the RF coil and the dielectric window. The shield comprises a planar body section having a periphery, central opening, and outer gaps forming a substantially continuous opening about the periphery. A uniform magnetic field, inductively coupled with the plasma, is formed by routing the flux lines of the magnetic field through the central opening and outer gaps of the shield. Contamination from sputtering is substantially eliminated by reducing the capacitive electric fields generated by the coil that interfere with the inductive coupling between the coil and the gas plasma.

This application is a division of application Ser. No. 08/005,678, filedJan. 19, 1993, (allowed).

TECHNICAL FIELD

This invention relates generally to the field of semiconductor devicemanufacturing, and specifically to plasma-based processes with reducedsputter contamination.

BACKGROUND ART

The uniform and rapid processing of materials using induction generated,plasma-based processes (also referred to as inductive-coupled plasmaprocesses) is important in the fields of semiconductor devicemanufacturing, packaging, optics, and the like. Many plasma processesare extensively used for the depositing or reactive etching of layersduring semiconductor device fabrication. However, the radio frequency(RF at about 13.56 MHz) induction plasma source is known to produce highelectron density (n_(e) >10¹¹ cm⁻³) plasmas, thus providing highprocessing rates.

One conventional apparatus described in U.S. Pat. No. 3,705,091 toJacob, produces a high density plasma which consists of a helical coilenergized by 13 MHz RF radiation. The plasma is generated inside a lowpressure cylindrical vessel within the helical coil. The coil structureinduces electric fields within the plasma region which drive thedischarge. High RF potentials on the coil cause capacitive coupling withthe vessel walls. The capacitive coupling accelerates charged particles(electrons and ions) from the plasma into the dielectric vessel wallscausing process contamination due to sputtering of the dielectric vesselwalls. In addition, capacitive coupling is much less efficient thaninductive coupling.

M. C. Vella et al. in Development of R.F. Plasma Generators for NeutralBeams, (Journal of Vacuum Science Technology, Vol. A3(3), pp. 1218-1221(1985)), describe an inductive-coupled plasma process having a coilimmersed in a plasma that is confined by permanent magnets. Thisapparatus also exhibits a degree of capacitive coupling to the dischargesince the coil is in contact with the plasma.

D. K. Coultras et al. in European Patent Application 0 379 828 and Oglein U.S. Pat. No. 4,948,458, describe inductive-coupled plasma processusing a spiral coil separated from the plasma by a planar dielectriccalled a window. Again, high potentials on the coil cause some degree ofcapacitive coupling, and thus contamination of the process due tosputtering of the dielectric window.

In U.S. Pat. No. 4,918,031, Flamm et al. describe a helical resonatorwith a coil similar to that of Jacob in which a split cylindrical groundshield is placed between the coil and the vacuum vessel such that highfields from the coil are shorted to ground. Capacitive coupling isessentially eliminated in this configuration. However, the cylindricalgeometry of this device does not allow efficient use of the ions andreactive species on large area substrates such as semiconductor wafers.Additionally, the cylindrical geometry can not be scaled for use withvery large area substrates such as liquid crystal displays.

What is desired is a technique for both eliminating capacitive couplingto reduce contamination, and maintaining high inductive coupling betweenthe coil and the plasma for improved processing rates as well as areactor geometry which is scalable to large areas.

DISCLOSURE OF INVENTION

The present invention is directed to a method for enhanced inductivecoupling to plasmas with reduced sputter contamination. The presentinvention eliminates sputtering of the dielectric window by shunting toground capacitive electric fields generated by high potentials on theadjacent spiral-like or helical coil. This is achieved by addinggrounded conducting elements, called shields, between the dielectricwindow and the coil.

The shields of the present invention are designed so that they do notinterfere with the inductive coupling of the coil to the plasma, butguide capacitive electric fields generated by the coil away from theplasma-window interface and toward ground.

The primary advantage of the present shielding invention is thereduction or elimination of sputtered contaminates from the dielectricvacuum window.

The shields also guide the induction electric field through the plasmain a way such that plasma generation uniformity is improved when helicalcoils are used.

Another advantage is improved generation of ions in the plasma. Thisimproved generation of ions in the plasma causes increased etch ratescompared to the rate achieved using conventional plasma-based processes.

The foregoing and other features and advantages of the invention will beapparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be better understood if reference is made to theaccompanying drawings.

FIG. 1 shows a schematic cross-section of a plasma-based semiconductordevice manufacturing apparatus of the present invention.

FIG. 2 shows representative drawings of the shields shown in FIG. 1.

FIGS. 3A and 3B are diagrammatic representations of magnetic flux linespassing around the conductive shield of the present invention.

FIGS. 4A and 4B show photographs of polyimide-coated wafers which wereetched under identical conditions both with and without capacitiveshielding.

FIG. 5 is a plot showing measured ion saturation current as a functionof diagonal position in the plasma with and without shielding.

FIG. 6 shows the data of FIG. 5 normalized to the same peak values.

FIG. 7 shows a further embodiment of the present invention adapted forhelical coils.

FIG. 8 shows electrically conducting shield for use with the helicalcoil embodiment.

FIG. 9 shows a diagonal uniformity comparison between a preferredembodiment of the present invention and a conventional spiral coil.

In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit of thereference number identifies the drawing in which the reference numberfirst appears.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a schematic cross-section of a plasma-based semiconductordevice manufacturing apparatus 100 of the present invention.

A general operational description of an inductive coupled plasmalow-pressure chemical vapor deposition (CVD) or reactive ion etching(RIE) apparatus is found in Coultras, et al., and Ogle, the disclosuresof which are incorporated herein by reference.

Referring now to FIG. 1 of the present invention, a low-pressure plasmaprocess chamber 102 comprises a substrate holder 104 for supporting aworkpiece 106. Other applicable processes include: plasma etching, CVD,surface treatment, atom and radical source, ion beam source and lightsource (visible, UV, vacuum UV). More specifically, the workpiece 106 isone or more semiconductor wafers, or the like. The process chamber 102has a process gas inlet 108 through which a process gas is pumpedaccording to conventional techniques. A plasma 110 is generated insidethe low-pressure process chamber 102, as will be discussed furtherbelow. The process chamber 102 also includes permanent magnets 112 whichare used for shaping the plasma 110 during processing.

Attached to the top of the process chamber 102 is an RF housing 114,which is commonly referred to as a "matchbox". Housed within thematchbox 114 is a spiral-shaped coil 116 and an RF impedance-adjustingcircuit 118. The RF impedance-adjusting circuit 118 is powered by an RFpower input 120. A quartz vacuum window (dielectric window) 122separates the RF coil 116 from the process chamber 102 and (duringoperation) the plasma 110. Also separating the RF coil 116 from theprocess chamber 102 is a conductive shield 124 and insulating layer 126(such as air, gas, vacuum, or the like) sandwiched between the RF coil116 and the conductive shield 124.

The spiral coil 116 is generally planar, and is therefore also referredto as a planar coil, but for purposes of this description will be simplyreferred to as the "coil 116." As evident by inspection of the drawing,coil 116 is located outside of the process chamber and inside of thematchbox 114. The coil 116 is positioned proximate the dielectric window122, but is separated therefrom by the insulating layer 126 andconductive shield 124. As discussed in Ogle, for example, the planargeometrical shape of coil 116 produces a planar plasma 110 for more evenprocessing of the workpiece 106. Thus, the plane in which the coil 116lies is substantially parallel to the dielectric window 122 andconductive shield 124. The conductive shield 124 is grounded byconnection directly to the matchbox 114, which in turn is connected toprocess chamber 102.

During operation of the apparatus the conductive shield 124 guidescapacitive electric fields generated by the coil 116 away from thedielectric window 122 and to the grounded matchbox 114. This groundingof the capacitive electric fields substantially reduces interference bythe capacitive electric fields with the inductive coupling between thecoil 116 and the plasma 110.

The basic geometry of conductive shield 124 will now be discussed inconnection with FIG. 2. In a preferred embodiment of the presentinvention, the conductive shield 124 comprises four shield elements 230.The shield elements 230 are made of conductive metal such as copper,aluminum, or the like, having a thickness on the order of about 0.01-1mm.

The shield elements 230 each comprise a ground lead 232 for connectionto an inside wall of the matchbox 114. Other equivalent techniques forgrounding the conductive shield 124 should become evident to thoseskilled in the art.

Each shield element 230 has an inner edge 234, two side edges 236, andan outer edge 238. The side edges 236 and an outer edges 238 define aperiphery of the shield 124.

A center opening in the conductive shield 124 is defined by theextremities of the four inner edges 234, as shown generally in FIG. 2.In addition, radial gaps 242 are defined by the side edges 236 ofadjacent shield elements 230. Finally, outer gaps 244 are defined by theouter edges 238 and the inside wall of the matchbox 114 represented by adashed line 246.

Representative dimensions of the shield elements 230 will now bediscussed with reference to FIG. 2. These dimensions are merelyrepresentative of a preferred embodiment of the present invention.Modifications can be made to the general shape of the capacitive shield124 and shield elements 230 without departing from the spirit and scopeof the present invention.

The shield elements 230 are separated by the radial gaps 242 so thatthere does not exist a completed circular conducting path which wouldprevent magnetic inductive fields from reaching the plasma region.

In a preferred embodiment, the distance between inner edge 234 and outeredge 238 is represented by the constant x. The lengths of the inneredges 234 and outer edges 238 are represented by the constants y and z,respectively. In this representative embodiment, y is equal to about 2x,and z is equal to about 2y. In this representative embodiment, an angleα between the inner edge 234 and side edge 236 is approximately equal to135°. With α equal to approximately 35°, the side edges 236 areapproximately 1.4x. FIG. 2 also shows a 90° bend at angle β forattaching the ground leads 232 of the shield elements 230 to theinterior wall of the matchbox 114. Again, these dimensions are onlyrepresentative examples of the invention and are not limitationsthereof.

The present embodiment is intended for uniform treatment of squaresubstrates. In the case of treatment of circular substrates theperiphery of a shield would form a circle. The shape of the shieldingmodifies the geometry of inductive electric fields for optimaluniformity over various shaped substrates (workpieces). Further, oneskilled in the art will recognize that the window, shielding, and coilneed not be planar. In the instance of treating dome-like workpieces, itis advantageous to use a domed or hemispherical window. The shieldingwould then preferably be domed or hemispherical and conformal to thewindow, but may flat. The coil may then be conformal to the shielding orhelical in shape.

FIGS. 3A and 3B are diagrammatic representations of the capacitiveshield 124 and RF magnetic flux lines 302 produced by the coil 116. FIG.3A is a cross-section of the coil 116 and the shield elements 230. Thisfigure also shows the center opening 240, which permits the RF magneticflux lines 302 to pass through the dielectric window 122 to generate theplasma 110 (both not shown). The outer gaps 244 permit the RF magneticflux lines 302 to return to the coil 116, as also shown in the figure.The shield elements 230 are grounded, as shown schematically at 304.FIG. 3B shows a top view of the capacitive shield 124. Again, the RFmagnetic flux lines 302 are shown entering the center opening 240 andreturning to the coil 116 via the outer gaps 244 and the radial gaps242.

Because the inner end of the coil 116 is at ground potential, the RFpotential on an inner turn or the coil 116 is very low. The shield'scentral opening 240 thus permits inductively coupling between the coil116 and the plasma 110 without concern for sputtering of the center ofthe dielectric window 122 by RF potentials. If, however, the centralopening is made too small (less than approximately 1 inch), plasmaignition becomes difficult and a portion of the induction field isexcluded from the plasma region by the shield.

FIGS. 4A and 4B show photographs of polyimide-coated wafers which wereetched under identical conditions both with and without the capacitiveshielding of the present invention. The unshielded process shown in FIG.4A produces a wafer which is hazed and rough on the surface.

This roughness is due to micromasking of the surface of the polyimide bysilicon sputtered from the quartz window.

With the grounded shield elements in place the polyimide on the waferremains smooth and reflective, as shown in FIG. 4B. Polyimide removalrates are faster with the conductive shield in place. The average etchrate of the wafer etched with the conductive shield of the presentinvention was 0.75 μm/min. Without shielding the etch rate underidentical conditions is 0.55 μm/min. While this increased etch rate maybe partly due to the elimination of micromasking, Langmuir probemeasurements of the ion flux from the plasma also show an increased ionsaturation flux density when shielding is used.

FIG. 5 plots the measured ion saturation current as a function ofdiagonal position in the plasma with and without shielding. Theshielding increases the ion flux by 50%. FIG. 6 shows the data of FIG. 5normalized to the same peak values to demonstrate that the ion fluxuniformity is not adversely affected by these shields.

It is evident by inspection of the physical results and data shown inFIGS. 4-6, that the present invention is effective at reducing sputtercontamination, while at the same time improving process rates in lowpressure plasma processes.

In another preferred embodiment of the present invention, theelectrically conducting shield is used to shape the RF field geometry ofa non-planar inductive coil such that uniformity is improved. Shaping ofthe conductive shield forces the induction field to be more uniformwithin the plasma even when a helical coil is used, for example.

An RF induction plasma and ion source using a helical coil designtogether with the electrically conducting shield of the presentinvention are shown in FIG. 7. Rather than generating a plasma withinthe coil, as taught by Jacob, a planar plasma 110 below a helical coil702 is generated using a grounded, electrically conducting shield 724 ofthe present invention.

Planar plasmas are desirable for treating planar workpieces such assilicon wafers and multi-chip packages. To improve uniformity the shapedconducting shields are used between the plasma and the end of the coilsuch that the RF fields are modified in shape to generate a morespatially uniform plasma. Thus, the present invention can be used foruniform plasma processing of large area materials. The presentembodiment was implemented for uniformity over square surface areas, andhence, this embodiment is optimized for square-shaped plasma excitation.However, the principal of this design is applicable to many othergeometries, as those skilled in the art will recognize.

FIG. 7 shows a low pressure (0.1-100 mTorr) plasma generated in a vacuumchamber 102. Radio frequency energy (13.56 MHz) is introduced by ahelical coil 702 powered by a supply 706, to the discharge regionthrough a quartz vacuum window 122 located at the top of the vacuumchamber 102. Both the chamber 102 and the end coil of helical coil 702are grounded. An intense magnetic field is generated by the helical coil702 which resides adjacent to the vacuum window 122 in the matchbox 114.The coil may consist of 1/4 inch copper tubing wound about an 8 inchdiameter coil form. FIG. 7 also shows that the substrate holder 104 forsupporting the workpiece 106 may be protected by a shield 708.

FIG. 8 shows electrically conducting shield 724 for use with the helicalcoil embodiment. The electrically conducting shield 724 is similar inoperation and structure to the shield 124 described above. Theelectrically conducting shield 724 comprises shield elements 830, groundleads 832, inner edge 834, side edge 836, outer edge 838, center opening840, radial gaps 842 and outer gaps 844.

Magnetic flux lines loop through the helix coil and pass through theplasma region inducing an electric field in the plasma. The fieldsgenerated by the coil alone are somewhat non-uniform. The uniformity isimproved by the grounded conducting shield between the coil and theplasma. The RF magnetic flux generated by the helical coil 702 is forcedthrough the center region of the plasma 110. The flux's return path isthen within the plasma and around the outside of the shaped conductingpieces of the shield. The shape of the fields, and hence the uniformityof plasma generation, is controlled by the shape of the conductingpieces. The shape of the coil is secondary, and may assume manyspiral-like geometries. For additional uniformity, those skilled in theart will recognize that magnetic confinement of the plasma may be used.

The grounded end of the helical coil in the present embodiment ispositioned near the plasma, thus capacitive electric fields between thecoil and the plasma are very small compared to those generated by aspiral coil, thus grounding of the conductive shield 724 may not benecessary.

The uniformity of plasma generated by the present embodiment is improvedover conventional spiral couplers as shown in FIG. 9. The diagonaluniformity of the spiral coupler over about 20 cm is 19%, but, underidentical conditions, a helical coil having a conducting shield of thepresent invention achieves 11% uniformity. The actual ion flux measuredis approximately the same for both devices at (i.e., about 20 mA cm⁻²).

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention. All cited articles and patent documents in theabove description are incorporated herein by reference.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is:
 1. A method of plasma treating a workpiecewith a uniform, high density plasma for enhanced processing and reducedsputter contamination, comprising the steps of:supporting the workpiecein a process chamber, the process chamber having a dielectric windowlocated in close proximity to the workpiece and a grounded, electricallyconducting shield, wherein said shield has a periphery and an outer gapforming a substantially continuous opening about the periphery;introducing a process gas into the process chamber; inducing a highfrequency current in a coil located outside the process chamberproximate to the dielectric window, wherein said high frequency currentin turn produces a magnetic field which generates a substantiallyuniform and dense plasma of said process gas inside the process chamberfor enhanced processing of the workpiece; guiding the magnetic fieldthrough the outer gap of said shield, thereby providing an efficientreturn path for the magnetic field and maintaining the uniformity anddensity of said plasma; and shielding capacitive electric fieldsgenerated by the coil away from the dielectric window and toward groundto substantially reduce interference by the capacitive electric fieldswith inductive coupling between the coil and said plasma, thussubstantially eliminating contamination from sputtering of thedielectric window by the capacitive electric fields.
 2. The method ofclaim 1, wherein said shield includes a central opening and radial gaps.3. A method of plasma treating a workpiece, comprising the stepsof:supporting the workpiece in a process chamber, the process chamberhaving a dielectric window located in close proximity to the workpieceand a grounded, electrically conducting shield, wherein said shield hasa periphery, a central opening, and outer gaps forming a substantiallycontinuous opening about the periphery; introducing a process gas intothe process chamber; inducing a high frequency current in a coil locatedoutside the process chamber proximate the dielectric window, whereinsaid high frequency current in turn produces a magnetic field havingflux lines which generates a plasma of said process gas inside theprocess chamber; establishing and maintaining the uniformity and densityof said plasma for enhanced processing of the workpiece by routing theflux lines of said magnetic field through the central opening and outergaps of said shield; and shielding capacitive electric fields generatedby the coil away from the dielectric window and toward ground tosubstantially reduce interference by the capacitive electric fields withinductive coupling between the coil and said plasma, thus substantiallyeliminating contamination from sputtering of the dielectric window bythe capacitive electric fields.
 4. The method of claim 3, wherein theestablishing and maintaining step further comprises:routing the fluxlines of said magnetic field through annular gaps which form asubstantially continuous opening approximately a third of the radialdistance from the central opening and the outer gaps.
 5. The method ofclaim 3, wherein the shield and the coil are substantially flat and thecoil is positioned substantially parallel to the dielectric window. 6.The method of claim 3, wherein the shield and the coil are substantiallyhemispherical in shape.
 7. The method of claim 3, wherein the coil ishelical in shape.
 8. A method of plasma treating a workpiece in aprocess chamber, the process chamber having a dielectric window, agrounded electrically conducting shield, and a coil, wherein the shieldhas a periphery, a central opening, and outer gaps forming asubstantially continuous opening about the periphery, and the coil islocated outside the process chamber proximate the dielectric window,comprising the steps of:supporting the workpiece in the process chamberin close proximity to the dielectric window and the coil; introducing aprocess gas into the process chamber; inducing a high frequency currentin the coil, wherein said high frequency current in turn produces amagnetic field having flux lines which generates a plasma of saidprocess gas inside the process chamber; establishing and maintaining theuniformity and density of said plasma by routing the flux lines of saidmagnetic field through the central opening and outer gaps of the shield,thereby enhancing the processing the workpiece; and shielding capacitiveelectric fields generated by the coil away from the dielectric windowand toward ground to substantially reduce interference by the capacitiveelectric fields with inductive coupling between the coil and saidplasma, thus substantially eliminating contamination from sputtering ofthe dielectric window by the capacitive electric fields.
 9. The methodof claim 8, wherein the establishing and maintaining step furthercomprises:routing the flux lines of said magnetic field through annulargaps which form a substantially continuous opening approximately a thirdof the radial distance from the central opening and the outer gaps. 10.The method of claim 8, wherein the shape of the shield substantiallyconforms with the shape of the workpiece for providing optimaluniformity over the workpiece.